avoid 32-bit relocation to __BOOTLOADER_CONFIG

As explained in my comment in rust-osdev#427, the compiler seems to have trouble
emitting relocations for references to global variables in custom
sections. For custom sections, it always emits 32-bit relocations even
when a 64-bit relocation would be required. This patch works around
that by never referencing the global in the custom section directly
from code, but only through a pointer from another global variable in
the non-custom .data section. The relocation used for the pointer in
the global variable will always use a 64-bit relocation.

Author: Freax13

api/src/lib.rs

@@ -120,13 +120,17 @@ macro_rules! entry_point {
                 config.serialize()
             };
 
+            // Workaround for https://github.com/rust-osdev/bootloader/issues/427
+            static __BOOTLOADER_CONFIG_REF: &[u8; $crate::BootloaderConfig::SERIALIZED_LEN] =
+                &__BOOTLOADER_CONFIG;
+
             #[export_name = "_start"]
             pub extern "C" fn __impl_start(boot_info: &'static mut $crate::BootInfo) -> ! {
                 // validate the signature of the program entry point
                 let f: fn(&'static mut $crate::BootInfo) -> ! = $path;
 
                 // ensure that the config is used so that the linker keeps it
-                $crate::__force_use(&__BOOTLOADER_CONFIG);
+                $crate::__force_use(__BOOTLOADER_CONFIG_REF);
 
                 f(boot_info)
             }